The present patent application claims the right of priority under 35 U.S.C. xc2xa7119 (a)-(d) and 35 U.S.C. xc2xa7365 of International Application No. PCT/EP99/09774, filed Dec. 10, 1999, which was published in German as International Patent Publication No. WO 00/39191 on Jul. 6, 2000, which is entitled to the right of priority of German Patent Application Numbers: 198 59 690.1, filed Dec. 23, 1998; 198 59 692.8, filed Dec. 23, 1998; and 199 04 408.2, filed Feb. 4, 1999.
The present invention relates to polycarbonates with low yellowness index (YI) and high transmission (TD) obtainable by the phase interface process, a process for the production thereof, in which sodium bisphenolate solutions with a dissolved oxygen content of  less than 150 ppb are used, a process for the production of such sodium bisphenolate solutions, and substrates for optical data storage media with improved transmission in the blue spectral region.
The synthesis of polycarbonates via the phase interface process is known. The objective is to produce polycarbonates with high transmission which are as colourless as possible. The yellowness index (YI) is a criterion of the inherent coloration of the polycarbonate, the transmission value (TD) a criterion of light transmission. The object of the invention was to provide polycarbonates with as low as possible a YI and a high TD.
It has now been found that such polycarbonates may be obtained when sodium bisphenolate solutions with a dissolved oxygen content of  less than 150 ppb, preferably  less than 100 ppb, particularly preferably  less than 50 ppb are used for the production thereof.
The invention therefore provides polycarbonates, obtainable by conversion of phosgene with a sodium bisphenolate solution with a dissolved oxygen content of  less than 150 ppb, preferably  less than 100 ppb, particularly preferably  less than 50 ppb, by the phase interface process, such as is described in DE-OS 42 27 372 for example, with the exclusion of oxygen. In the configurational loop comprising reaction vessel and tubular reactor, recirculating loop and tubular reactors are flooded and the reaction vessel superimposed with nitrogen so that exclusion of oxygen is ensured.
These polycarbonates are particularly suitable for producing optical data storage media, such as compact discs (CDs), magneto-optical discs (MODs) or DVDs which are written or read with short-wave laser light ( less than 500 nm, preferably 400 to 450 nm).
The invention also provides a process for the production of such polycarbonates by the phase interface process, in which phosgene is converted with a sodium bisphenolate solution with a dissolved oxygen content of  less than 150 ppb, preferably  less than 100 ppb, particularly preferably  less than 50 ppb, with exclusion of oxygen.
The invention further provides a process for the production of sodium bisphenolate solutions with a dissolved oxygen content of  less than 150 ppb, preferably  less than 100 ppb, particularly preferably  less than 50 ppb, in which bisphenols with a dissolved oxygen content of  less than 10 ppb are converted with an aqueous NaOH solution with a dissolved oxygen content of  less than 100 ppb with exclusion of oxygen.
Bisphenols which may be used according to the invention are those which are obtainable by conversion of aromatic hydroxy compounds which are not substituted in the p-position and contain no second-order substituents such as cyano, carboxy or nitro groups, for example phenol, o- and m-cresol, 2,6-dimethylphenol, o-tert.-butylphenol, 2-methyl-6-tert.-butylphenol, o-cyclohexylphenol, o-phenylphenol, o-isopropylphenol, 2-methyl-6-cyclopentyl-phenol, o- and m-chlorophenol, 2,3,6-trimethylphenol, preferably phenol, o- and m-cresol, 2,6-dimethylphenol, o-tert.-butylphenol and o-phenylphenol; particularly preferably phenol, and ketones with at least one aliphatic group at the carbonyl function, for example acetone, methylethylketone, methylpropylketone, methylisopropylketone, diethylketone, acetophenone, cyclohexanone, cyclopentanone, methyl, dimethyl and trimethyl cyclohexanones, which may also have geminal methyl groups, e.g. 3,3-dimethyl-5-methylcyclohexanone (hydroisophoron), preferably acetone, acetophenone, cyclohexanone and the homologues thereof having methyl groups; particularly preferably acetone. Intensive nitrogen inertization during the production process ensures that the residual dissolved oxygen content in the bisphenols is less than 10 ppb.
The bisphenols are converted under exclusion of oxygen (nitrogen inertization) with aqueous NaOH solutions which have a dissolved oxygen content of  less than 100 ppb, preferably  less than 20 ppb. The concentration of the aqueous NaOH solution is preferably selected in such a way that the concentration of the resulting sodium bisphenolate solution is as close as possible to the limit of solubility, i.e. in the range from 13 to 16 wt. %, preferably 14 to 15.5 wt. %. The molar ratio of NaOH to bisphenol is 1.8:1 to 2.5:1, preferably 1.9:1 to 2.4:1, particularly preferably 2.0:1 to 2.3:1. The bisphenol may be dissolved in the NaOH as solid although it is preferably added to the NaOH direct as melt without having passed through the solid state so that solutions are obtained with a temperature of 20xc2x0 C. to 90xc2x0 C., preferably 30xc2x0 C. to 70xc2x0 C.
The aqueous, virtually oxygen-free NaOH used to produce the sodium bisphenolate solution may be produced by electrolysis. Storage and transport of the NaOH after production must take place under inert gas. For use in the process according to the invention the concentration of the NaOH obtained by electrolysis is generally lowered by dilution with virtually oxygen-free, fully demineralized water. The fully demineralized water is freed from oxygen in a manner known in principle, e.g. catalytically, by degassing or stripping with inert gas.
The sodium bisphenolate solutions produced by the process according to the invention have particularly low colour values which naturally also depend on the colour value of the bisphenol used. Where a bisphenol with a colour value of  less than 10 Hz is used, colour values of  less than 1.5 Hz, preferably  less than 1.0 Hz, can be achieved.
The sodium bisphenolate solutions produced by the process according to the invention also exhibit distinctly improved storage stability. A sodium bisphenolate solution having an oxygen content of 20 ppb and stored over 3 hours at 40xc2x0 C. in inert gas conditions thus exhibits a colour value deterioration of only 0.5 Hz, whereas a colour value deterioration of 3.5 Hz is observed in a solution with an oxygen content of 250 ppb.
The invention further provides the substrates for optical data storage media which may be produced with the polycarbonates according to the invention and the optical data storage media which may be produced from the substrates.
To be able to achieve higher storage densities in optical data storage media, it is intended to replace the read/write systems which have been used hitherto and operate with light in the red spectral region with those which use wavelengths in the blue spectral region, specifically from 400 to 450 nm wavelength. For this purpose the substrates used to produce the optical data storage media are required to exhibit the highest possible transmission in this wavelength range. This is crucial to a good signal-to-noise ratio and a long service life of the data carrier.
It has now been found that substrates with the desired properties may be produced from polycarbonate which was produced using sodium bisphenolate solutions with a dissolved oxygen content of  less than 150 ppb, preferably  less than 100 ppb, particularly preferably  less than 50 ppb.
The invention provides substrates for optical data storage media comprising polycarbonate which is obtainable by conversion of phosgene with a sodium bisphenolate solution with a dissolved oxygen content of  less than 150 ppb, preferably  less than 100 ppb, particularly preferably  less than 50 ppb, by the phase interface process, such as is described in DE-OS 42 27 372 for example, with exclusion of oxygen.
The substrates according to the invention may be produced from homo or copolycarbonates. Bisphenol A polycarbonate or copolymers based on bisphenol A and bisphenol TMC or bisphenol M and bisphenol TMC are preferably used.
In the configurational loop comprising reaction vessel and tubular reactor, recirculating loop and tubular reactors are flooded and the reaction vessel superimposed with nitrogen so that exclusion of oxygen is ensured.
These substrates are used for the production of optical data storage media, such as compact discs (CDs), magneto-optical discs (MODs) or digital versatile discs (DVDs and/or high-density DVDs) which are written and/or read with short-wave laser light ( less than 500 nm, preferably 400 to 450 nm). The invention also provides the optical data storage media produced from the substrates, which media are produced from the substrates in a manner familiar to the person skilled in the art in principle.
In the production of the polycarbonate for the substrates according to the invention, sodium bisphenolate solutions with a dissolved oxygen content of  less than 150 ppb, preferably  less than 100 ppb, particularly preferably  less than 50 ppb, are used, which are obtainable by conversion of bisphenols with a dissolved oxygen content of  less than 10 ppb with an aqueous NaOH solution with a dissolved oxygen content of  less than 100 ppb, with exclusion of oxygen.
Bisphenols which may be used according to the invention are those which are obtainable by conversion of aromatic hydroxy compounds which are not substituted in the p-position and contain no second-order substituents such as cyano, carboxy or nitro groups, for example phenol, o- and m-cresol, 2,6-dimethylphenol, o-tert.-butylphenol, 2-methyl-6-tert.-butylphenol, o-cyclohexylphenol, o-phenylphenol, o-isopropylphenol, 2-methyl-6-cyclopentyl-phenol, o- and m-chlorophenol, 2,3,6-trimethylphenol, preferably phenol, o- and m-cresol, 2,6-dimethylphenol, o-tert.-butylphenol and o-phenylphenol; particularly preferably phenol, and ketones with at least one aliphatic group at the carbonyl function, for example acetone, methylethylketone, methylpropylketone, methylisopropylketone, diethylketone, acetophenone, cyclohexanone, cyclopentanone, methyl, dimethyl and trimethyl cyclohexanones, which may also have geminal methyl groups, e.g. 3,3,5-trimethyl cyclohexanone (hydroisophoron), preferably acetone, acetophenone, cyclohexanone and the homologues thereof having methyl groups; particularly preferably acetone. 1,3-bis[1-(4-hydroxyphenyl)-1-methylethyl]benzene (bisphenol M, CAS No. 13595-25-0) may also be used as bisphenol. The production thereof is described in U.S. Pat. No. 5,633,060. Intensive nitrogen inertization during the production process ensures that the residual dissolved oxygen content in the bisphenols is less than 10 ppb.
The bisphenols are converted under exclusion of oxygen (nitrogen inertization) with aqueous NaOH solutions which have a dissolved oxygen content of  less than 100 ppb, preferably  less than 20 ppb. The concentration of the aqueous NaOH solution is preferably selected in such a way that the concentration of the resulting sodium bisphenolate solution is as close as possible to the limit of solubility, i.e. in the range from 13 to 16 wt. %, preferably 14 to 15.5 wt. %, in the case of bisphenol A. The molar ratio of NaOH to bisphenol is 1.8:1 to 2.5:1, preferably 1.9:1 to 2.4:1, particularly preferably 2.0:1 to 2.3:1. The bisphenol may be dissolved in the NaOH as solid although it is preferably added to the NaOH direct as melt at temperatures of 20xc2x0 C. to 90xc2x0 C., preferably 30xc2x0 C. to 70xc2x0 C. without having passed through the solid state. Mixtures of different bisphenols may, of course, also be used, such as mixtures of bisphenol A and bisphenol TMC (1,1-bis[4-hydroxyphenyl]-3,3,5-trimethylcyclohexane).
The aqueous, virtually oxygen-free NaOH used to produce the sodium bisphenolate solution may be produced by electrolysis. Storage and transport of the NaOH after production must take place under inert gas. For use in the process according to the invention the concentration of the NaOH obtained by electrolysis is generally lowered by dilution with virtually oxygen-free, fully demineralized water. The fully demineralized water is freed from oxygen in a manner known in principle, e.g. catalytically, by degassing or stripping with inert gas.
The sodium bisphenolate solutions obtained in this way have particularly low colour values which naturally also depend on the colour value of the bisphenol used. Where a bisphenol with a colour value of  less than 10 Hz is used, colour values of  less than 1.5 Hz, preferably  less than 1.0 Hz, can be achieved.